For persons who lie on a planar surface, it is widely known and conventional to lift the head and torso temporarily from the horizontal position by pushing objects such as pillows beneath the head or else beneath the torso. In more comfortable variants the upper portion of the bed frame can be lifted for this purpose. Thus, German patent DE 1 9 1 40 describes a bed, the upper frame portion of which can be pivoted about an axis oriented transversely to the longitudinal direction of the bed. The special feature in this case is that, by means of a hydraulic pump and a hydraulic cylinder, the upper frame portion can be lifted by the reclining person himself into an inclined position.
With the much larger number of adjustment possibilities, German patent 94 07 424 T2 describes a bed for the hospital sector, which is subdivided along its length into numerous transversal sections, which can be adjusted in their level and inclination. In addition, the adjustment elements are arranged in two groups on a carriage, which is displaceable in the longitudinal direction on an overall frame.
In particular with very aged people or patients with restricted mobility—for example with muscle shrinkage or other weakening—the lifting of the head and torso by pivoting the upper frame portion about a transverse axis leads to the head, or even the trunk sliding to the longitudinal side of the bed. In an extreme case, the caregiver must intervene manually to prevent the patient falling laterally out of the bed.
Because the caregiver actuates the adjustment mechanism of the upper frame portion with one hand, only one hand is available for supporting the patient. If this hand does not lie with its entire surface of the patient, then, in the case of patients with ageing or weakening connective tissue, this may lead to haematomas.
The effect of lateral rolling off is readily understandable by considering, instead of the patient, a cylindrical body without any drive, which is mounted on the upper frame portion in the longitudinal direction. If, due to lifting of the upper frame portion, an oblique plane is crated, the cylindrical body must be oriented with high precision exactly perpendicularly to the transverse axis about which the upper frame portion is pivoted. If the cylindrical body deviates only slightly from this exact perpendicular, then, due to the lifting on the oblique plane, a force component is produced that is oriented transversely to the longitudinal axis of the cylindrical body and moves it in a lateral direction. If the cylindrical body starts to roll, then the force acting laterally on it is increased and moves the cylindrical body away from a perpendicular towards the pivot axis, as a result of which the transversely-directed force further increases, so that the cylindrical body rolls off laterally.
During raising of the upper frame portion, the torso lying thereon is thus in an unstable position. Normal people, or at least people of restricted mobility, can compensate for the transverse force caused thereby, so that they remain on the upper frame portion. However, that is impossible for people with back muscles lastingly weakened by age or illness.
The same applies to a lower frame portion that has been pivoted to an inclined position in order to lift a patient's legs. In this case, too, forces oriented transversely to the leg are exerted if the leg is not oriented very precisely perpendicular to the pivot axis of the lower frame portion. These transverse forces can force a person of very greatly handicapped motor functions outwards and out of the bed.